A semiconductor element of a type in which an energization path is formed in a vertical direction of the element for the purpose of coping with a high voltage and a large current is generally called a power semiconductor element (for example, an insulated gate bipolar transistor (IGBT), a metal oxide semiconductor field effect transistor (MOSFET), a bipolar transistor, or a diode). A semiconductor device in which the power semiconductor element is mounted on a circuit board and packaged with a filler is used in various fields, such as industrial equipment, vehicles, and railways. In recent years, along with increase in performance of equipment having the semiconductor device mounted thereon, there is more demand for increase in performance of the semiconductor device, such as increase in rated voltage and rated current or enlargement of a usage temperature range (to higher temperature or lower temperature).
A mainstream packaging structure of the semiconductor device is called a case-type packaging structure. The semiconductor device using the case-type packaging structure has the following structure. The power semiconductor element is mounted on a heat-radiating base plate through the intermediation of an insulating circuit board. The insulating circuit board includes a front-face electrode pattern on one face of an insulating substrate, and a back-face electrode pattern on another face of the insulating substrate. A case is caused to adhere to the base plate. Further, the semiconductor element mounted in the semiconductor device is connected to main electrodes. Bonding wires are used for the connection between the semiconductor element and the main electrodes. For the purpose of preventing insulation failure that occurs when a high voltage is applied, in general, an insulating gel-like filler as typified by a silicone gel is used as the filler of the semiconductor device.
In general, the amount of gas that can be dissolved in the silicone gel is decreased as the temperature is increased. Therefore, when the usage temperature range of the semiconductor device is enlarged and the silicone gel is used at higher temperature, the gas that has become insoluble in the silicone gel forms air bubbles. In a portion in which such air bubbles are generated, the insulation filling effect by the silicone gel cannot be obtained, and hence the insulation performance of the semiconductor device is degraded.
Further, the semiconductor device incorporates many members such as a semiconductor element, a bonding material, and a wire. Even when degassing processing is performed when the silicone gel is injected, there is a risk that air bubbles may be generated in the semiconductor device.
Further, those air bubbles in the silicone gel are assumed to be increased in size due to influences of external environments such as moisture absorption and heating.
As a countermeasure fora case in which air bubbles are generated in the silicone gel as described above or a case in which the silicone gel is separated from various members, it is conceivable to inject a rubber material in which air bubbles cannot be generated or grown into the material when the semiconductor device is used. The rubber material is injected into a narrow gap space between a surface portion at which a ceramic substrate of the insulating circuit board, for which an insulation property is required to be secured, is exposed and the base plate to which the insulating circuit board is bonded.
As a related-art semiconductor device, there is disclosed a semiconductor device having a structure in which a peripheral edge portion of the insulating circuit board is covered with a silicone rubber adhesive, which has a strong adhesive property with respect to the insulating substrate and a container bottom face, and then a filler is injected (for example, Patent Literature 1).